Japan's Ueno won gold in Athens four years ago
Japan's Masae Ueno took just 40 seconds to defeat Cuba's Anaysi Hernandez in the 70kg class to become only the third woman to retain her Olympic judo title.
The diminutive 29-year-old attacked with aggression, throwing her opponent over for an ippon and automatic win.
American Ronda Rousey won bronze, scoring early with a yuko and holding on over Germany's Annett Boeham.
Edith Bosch of the Netherlands also claimed bronze with an ippon throw to defeat Spain's Leire Iglasias.
Rousey's medal was the first for the US in women's judo since the competition was incorporated into the Games in Barcelona 1992.